The present invention relates to a sheet processing apparatus which prints or coats the two surfaces, obverse, and reverse of a sheet.
Conventionally, as shown in Japanese Patent Laid-Open No. 2003-182031, a sheet processing apparatus has been proposed which comprises a blanket cylinder which opposes the last impression cylinder of a printing unit and receives a sheet from the last impression cylinder, a lower blanket cylinder which opposes the blanket cylinder in the upstream sheet convey direction of a position where the blanket cylinder opposes the last impression cylinder, and an upper blanket cylinder which opposes the blanket cylinder in the downstream sheet convey direction of the opposing point where the blanket cylinder opposes the last impression cylinder and supplies varnish to the surface of the sheet. As shown in Japanese Utility Model Registration No. 2,585,995, a sheet processing apparatus has been proposed in which an eccentric bearing supports a blanket cylinder opposing an impression cylinder and a cylinder throw on/off mechanism pivots the eccentric bearing to throw on/off the blanket cylinder.
In the conventional sheet processing apparatuses described above, when transferring a sheet from the last impression cylinder to the blanket cylinder, if the sheet is scratched depending on the thickness or material of the sheet to be processed, the packing combination of the blanket cylinder is changed to change the gap amount between the circumferential surfaces of the last impression cylinder and blanket cylinder. Accordingly, each time the sheet type changes, the packing combination of the blanket cylinder must be changed, which requires time. This increases the load to the operator to degrade the productivity.
When the packing combination of the blanket cylinder changes, the printing pressure between the blanket cylinder and upper blanket cylinder and that between the blanket cylinder and lower blanket cylinder change to degrade the printing quality. To prevent this, the eccentric bearings of the upper and lower blanket cylinders are pivoted, thus adjusting the printing pressures of the upper and lower blanket cylinders. As this adjustment must be performed repeatedly while checking the quality, a large amount of paper is wasted. Also, this adjustment must be performed each time the packing combination of the blanket cylinder changes, requiring time.